# Poiseuille's Law question

1. Oct 12, 2005

### je55ica7

The pulmonary artery, which connects the heart to the lungs, has an inner radius of 2.8 mm and is 8.5 cm long. If the pressure drop between the heart and lungs is 400 Pa, what is the average speed of blood in the pulmonary artery?

I am using the equation pi*r^4(p1-p2)/(8*viscosity*length)

Would the p1-p2 be -400N or 400N? I thought a speed couldn't be negative... but it says "pressure drop". Would the answer be in m/s?

I'm lost.

2. Oct 12, 2005

### mcah5

It depends on which way you choose to be positive, and which way to be negative. If you choose from the heart to the lungs to be positive, the p1-p2 = 400 where p1 is the pressure at the heart and p2 is the pressure at the lungs. Then your flow rate would be positive. You could also choose from the lungs to the heart to be positive, and in that case p1 is the pressure at the lungs and p2 is the pressure at the heart, so p1-p2 = -400. Then your flowrate would be negative.